During total joint arthroplasty, instrumentation is used by the surgeon to align various cutting or milling guides relative to the bone and the natural joint line. This instrumentation is well known in the art and need not be discussed in any detail here. In general, there are two classes of instruments for use during surgery, intramedullary and extramedullary. Extramedullary instrumentation is design to be position outside of the patients bone and is usually affixed to the patients limb by a series of straps or pins. Intramedullary devices are designed to be positioned directly within the medullary canal of the bone and generally include an elongate rod which is pushed into the medullary canal. Since the intramedullary instruments are positioned within the bone to receive the joint, they are generally considered more accurate than extramedullary instrumentations.
However, one draw back to the use of intramedullary instrumentation is the potential to cause fat cells within the medullary canal to be displaced during insertion of the intramedullary rod of the instrument. This issue becomes especially problematic if a bi-lateral knee procedure is being performed which would require an intramedullary rod be inserted into the medullary canal of both femurs and both tibial. To reduce the concerns discussed above, surgeons often choose to use extramedullary instrumentation or to perform surgery on one knee at a time. By delaying a needed surgery on the other malfunctioning knee, the patient is required to undergo two separate surgery sessions.
The use of stemmed provisional implant to test the fit and position of the final implant also may further add to the build up of pressure within the femoral canal. It would not be unusual for a stemmed tibial and femoral provisional components to be inserted into the intramedullary canals of the prepared tibia and femur multiple times during a surgery to test the fit and anatomic functionality of the final implant. The stemmed components may create a build up of pressure within the intramedullary canal. As it is impossible to replicate these components in an extramedullary solution, the surgeon may be forced to delay the operation on the patients second knee if stemmed components will be required.